Kessel is near a black hole in the Star Wars galaxy. Since Han is a smuggler he has to sometimes find creative ways to outrun the law. The Kessel run was a route that was considered dangerous because it passed by the black hole. In order to lose the ships following them smugglers would cut close to the black hole in hopes of the other ships being sucked in. In order to not be sucked in themselves the smugglers would have to be considerably fast to overcome the gravity. Han cut it super close by coming within 12 parsecs from the black hole. So the Millennium Falcon did make the Kessel run in less than 12 parsecs.